Ferrofluid seals for the sealing of single rotary shafts either through the use of a single- or multiple-stage ferrofluid O-ring seal is a well founded and accepted technology. The effective sealing of two or more coaxial shafts in a single housing is quite difficult. Historically the solution to coaxial shaft sealing has been to mount physically one complete seal assembly inside another in order to de-couple the multiple magnetic structures. This approach to ferrofluid sealing of coaxial multiple shafts leads to an excessively large overall assembly which is quite cumbersome, particularly in use in small coaxial shaft diameters. Ferrofluid seals on coaxial, multiple-shaft elements are most desirable since such ferrofluid seals reduce wear on the sealing elements, provide for low drag torque, and may be designed for long ferrofluid seal life.
Typically, coaxial, multiple-shaft apparatus requiring effective sealings are used, for example, in semiconductor processing equipment, such as in sputtering assemblies for the etching or depositing onto a semiconductive wafer, and wherein a shutter, that is a plate with a hole, must be rotated to open and expose a target to a semiconductive wafer on a rotatable table below the target so that the rotating shutter is between the target and the table with the waffer. Another application for coaxial, multiple-shaft devices is for sample manipulation in certain high-vacuum analysis equipment, such as a goniometer, or for the use of x-ray equipment for crystallographic studies. Coaxial seals may be used with other devices wherein an effective seal is required, typically between a subatmospheric and an atmospheric or superatmospheric environment, and the seal is required between coaxial, multiple-shaft elements extending between the environments.
Thus, a seal apparatus for the hermetic sealing of the rotary motion of two or more coaxial shaft elements and for the independent rotation of the shafts in a low cost, compact design employing a ferrofluid seal is desirable.